title: Highly-accelerated Bloch-Siegert |B1+| mapping using joint autocalibrated parallel image reconstruction.
abstract: OBJECTIVE
  To reconstruct accurate single- and multichannel Bloch-Siegert transmit radiofrequency (|B(1)(+)|) field maps from highly accelerated data.
  
  
  METHODS
  The approach is based on the fact that the |B(1)(+)|-to-phase encoding pulse for each transmit coil and off-resonance frequency applies a unique phase shift to the same underlying image. This enables joint reconstruction of all images in a Bloch-Siegert acquisition from an augmented set of virtual receive coils, using any autocalibrated parallel imaging reconstruction method.
  
  
  RESULTS
  Simulations with an eight channel transmit/receive array head coil at 7T show that accurate |B(1)(+)| maps can be produced at acceleration factors of 16× and 6× for Cartesian and spiral sampling, respectively. A phantom experiment with a six channel transverse electromagnetic (TEM) transceive array coil allowed accurate reconstruction at 16× acceleration. 7T in vivo experiments performed using 32 channel receive and two-channel transmit coils further demonstrate the proposed method's ability to produce high-quality |B(1)(+)| maps at accelerations of 32× and 8× for Cartesian and spiral trajectories, respectively. Reconstruction accuracy is improved using disjoint k-space sampling patterns between acquisitions.
  
  
  CONCLUSIONS
  The proposed approach allows high acceleration factors in Bloch-Siegert |B(1)(+)| mapping and can significantly reduce the scan time requirements for mapping the |B(1)(+)| fields of transmit arrays.
authors.affiliation: Vanderbilt University Institute of Imaging Science, Nashville, Tennessee, USA; Department of Biomedical Engineering, Vanderbilt University, Nashville, Tennessee, USA.
authors.forename: Anuj
authors.initials: A
authors.lastname: Sharma
authors.forename: Sasidhar
authors.initials: S
authors.lastname: Tadanki
authors.forename: Marcin
authors.initials: M
authors.lastname: Jankiewicz
authors.forename: William A
authors.initials: WA
authors.lastname: Grissom
mesh_terms: D000465:Algorithms; D001921:Brain / Q000033:anatomy & histology*; D002138:Calibration; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
keywords: Bloch-Siegert acquisition; auto-calibrated; disjoint sampling; image reconstruction; parallel transmission